12BY7+2x6DQ6 Mopa Transmitter

I wanted to try an evolution of the commonly described as 'novice transmitters', many times found in the ARRL Handbooks or QSTs.

It's a general coverage TX from 3 to 30 MHz, 120 Wrms output, with electronic antenna relay (6AH6, from W1CP schematic). It features a fixed biased P.A., set at just the beginning of the tubes cutoff. This way, the full Osc. output voltage swing can exploited, in such a way that by properly setting the drive level it is possible both class 'B' (90W rms out, @ 0 mA drive) or class 'C' (120W rms out, @ 4 mA drive, and at about 150 Wrms input), providing an efficiency, in the last case, that reaches the 75 % !

The oscillator plate feeds a L-C tank in series to its B+ and a Neutralization capacitive divider is also used, as there is no self-biasing of the finals by the usual 27 .. 47 kOhm resistor, to contribute taming the P.A. restlessness ... :

The Neut. network has been arranged in a quick and dirty way, as shown in the above picture. It's a temporary solution, but works great.

The final tubes fixed bias, at just the onset of cut-off, allows to reach up to 8 ... 10 mA drive at the crystal fundamental frequency and at the peak of resonance, so there is plenty of margin for tripling or even qadrupling the frequency. Keying is at the cathodes and the note is smooth and clean. The electronic antenna relay allows a very comfortable and silent operation.

I'm very satisfied ! The 'novice MOPA' concept was extended to higher powers with more or less the same kind of components adopted at those times. The chassis is a salvaged 'BC-191' Tuning Unit (see the '1944' stamp in the above picture).

The most surprizing achievement of this TX has definitely been the P.A. efficiency. The credits go to the intrinsic features of the '6DQ6', which, to me, is a superb tube (same as 6JB6 used in the TR-4C or T-4C), featuring a wide cathode (at 1.2 A filament current !!), which can develop huge impulsive electron flows when required. The 6DQ6 is very robust too, plenty, relatively cheap and to my tastes .. more robust than the 6146. The 6DQ6 tolerates abuses that, otherwise, would almost kill an expensive 6146. A couple of '807' in the same circuit would not have performed such beautifully too.

The fixed bias, besides, has allowed me to proof that any tube in the class of the 12BY7 is more than abundant to get sufficient drive when going at the crystal 3rd or even 4th harmonic.

I will friendly call this transmitter 'The fingers killer' as its tremendous performance resulted also in this ...

77 de Cris

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